Fiber-optic current sensors commonly rely on the Faraday effect in fused silica fibers. The Faraday-effect varies with temperature. The Verdet constant V of fused silica fiber, which is a measure for the Faraday effect, changes according to (1/V)∂V/∂T=7×10−5° C.−1, i.e. within a temperature range of operation of e.g. −40° to +80° C. the sensor signal varies within 0.84%. Many applications of the sensor require accuracy to within ±0.2% or ±0.1%, however, and therefore require measures for temperature compensation. In EP 1107029, EP 1115000 and K. Bohnert, P. Gabus, J. Nehring, H. Brändle, “Temperature and Vibration Insensitive Fiber-Optic Current Sensor”, J. Lightwave Technol., 20(2), 267, (2002) a method was described for inherent temperature compensation of the Faraday effect in interferometric Sagnac and reflection-type fiber-optic current sensors. The method of inherent compensation eliminates the need of an extra temperature sensor, which is particularly important for current sensing at high electric potentials. The method exploits the temperature dependence of the fiber-optic retarder which generates the normally circular light waves propagating in the sensing fiber. For temperature compensation the retardation is set to a value which differs by a non-zero amount ε from the conventional 90°-retardation. The variation of the retardation with temperature affects the scale factor of the sensor. At the properly chosen retardation, e.g. with ε=10°, the influence of the retarder on the sensor sensitivity (scale factor S) just balances the variation of the Verdet constant with temperature.
In WO 2005/111633 and K. Bohnert, P. Gabus, J. Nehring, H. Brändle, M. Brunzel, “Fiber-Optic Current Sensor for Electrowinning of Metals”, J. Lightwave Technol., 25(11), 3602, (2007) it was shown that the sensor scale factor S is also influenced by the angle between the normal of the fiber coil plane and the slow axis of the polarization maintaining fiber before the retarder, called azimuth angle β=45°-β′, with β′ being the angle between the normal of the fiber coil plane and the slow axis of the retarder. The sensing fiber coil consists of a single loop of non-annealed single-mode fiber with a large loop radius and thus small bend-induced linear birefringence. The fiber resides in a capillary with friction reducing means and was packaged in a flexible strip of fiber reinforced epoxy. The variation of the scale factor S with the azimuth angle β is sinusoidal and varies within 0.8% of a retarder with ε=10° and a birefringent retardation in the sensing fiber of 1.5°. Hence, to achieve a stable scale factor, the azimuth angle must be controlled and fixed. In WO 2005/111633 preferred azimuth angles of β=0° (modulo 90°) and β=45° (modulo 90°) are said to achieve minimum sensitivity of the scale factor to variations of the bend induced birefringence and the azimuth angle, respectively. The effects of the temperature dependence of the bend induced birefringence [3] were neglected as they were very small in that case. However, a sensor fiber coil is conceivable and consists of several (non-annealed) fiber windings of small diameter. Such coils may be used, e.g. for integration into high-voltage (HV) equipment. In coils of this type a larger amount of fiber birefringence may be present, causing a birefringent phase retardation of e.g. δ=5 . . . 25°.
In EP 0856737 and K. Bohnert, P. Gabus, J. Nehring, H. Brändle, “Temperature and Vibration Insensitive Fiber-Optic Current Sensor” J. Lightwave Technol., 20(2), 267, (2002) a method of high temperature annealing was explained to efficiently reduce the bend induced birefringence in small diameter fiber coils with several fiber windings. The fiber of such coils may again reside in a glass capillary both during the annealing procedure and in the final sensor configuration. Depending on the coil diameter and the number of windings some birefringence may remain in the sensing fiber after annealing. The corresponding birefringent phase shifts can be of the order of several degrees. Furthermore, in a production environment the birefringence may vary from coil to coil to some extent. Variations in the birefringence as a result of tolerances in the production process and/or due to temperature variations affect the above mentioned temperature compensation of the Faraday effect. This is true both for annealed and non-annealed coils.